- A carefully drafted contract, outlining each party's rights and responsibilities, covering as many what-ifs as possible
- A horse that is compatible with your skills and riding style
That's all I ever had, never had a problem (with the horse).
And please, do not think of enjoying your own horse (singular) on your property. They are herd animals. Think about whether you can give any horse you own living conditions which allow it to thrive, and put your convenience secondary. There are 168 hours in the week, of which the horse will sleep about 28, if that. Of those 140 waking hours, you'll spend maybe 2-5 interacting with the horse. During the remaining time, the horse will long for the safety of a herd, space to move (ability to move means safety), and social interaction with members of its own species.
I rode a Percheron on the trail a few times - that canter is something else...like riding a cruise ship on land! (Turns like one, too!)